Method when cleaning a filter

ABSTRACT

When cleaning a barrier filter, comprising a plurality of filter elements, of woven fabric of felt, arranged to separate particles out of a polluted gas, the filter elements are cleaned, separately or in groups, by pressurized air pulses, the frequency, the maximum pressure and the duration of which are varied in order to minimize, by an adjustment, the total emission of dust. The frequency and/or the maximum pressure and/or the duration of the cleaning pulses are varied for a group or a plurality of groups of filter elements. After each cleaning pulse, the maximum value for the instantaneous emission of dust, the emission peak, is determined and the emission peak is used, after cleaning a certain group of filter elements, for selecting the frequency and/or the maximum pressure and/or the duration of the cleaning pulses for this group of filter elements during continued operation.

TECHNICAL FIELD

[0001] The present invention refers to a method for cleaning a barrierfilter, comprising a plurality of filter elements, of woven fabric orfelt, arranged to separate particles out of a polluted gas. The filterelements are cleaned, separately or in groups, by pressurized airpulses, the frequency, the maximum pressure, and the duration of whichcan be varied in order to minimize, by an adjustment, the total emissionof dust and maximise the life of the filter elements.

[0002] The method is in particular intended for optimising the cleaningof textile barrier filters, having filter elements in the form of tubesmanufactured of woven fabric or felt.

TECHNICAL BACKGROUND

[0003] When separating particulate pollutions out of a streaming gas,one of the most common methods of cleaning is letting the gas streamthrough a medium, on the surface of which or in the interior of whichthe particles are deposited. A general term for these filters is barrierfilters. Barrier filters can in principle be built-up of almost allthinkable solid materials, but the build-up principles are commonlyeither a rigid medium, such as a porous ceramic or a gravel bed, or aflexible medium, such as a woven fabric or felt.

[0004] During operation, particles are accumulated on the filter mediumand a dust heap is built up. This provides an increased flow resistanceand consequently leads to an increased pressure drop over the barrierfilter. In the long run, it can completely clog the filter medium. Asecure operation requires exchanging or cleaning of the filterelements/filter medium, either on site or by being taken out and, forinstance, washed or brushed. To purify gases having a low particlecontent, disposable filters or filters which are taken out and cleanedwere frequently used, and to purify gases having a high particlecontent, barrier filters which are cleaned on site were frequently used.

[0005] The cleaning on site can be done in several ways. In smallerplants it can, for instance, be carried out by movable suction nozzles,but in plants for purifying large gas flows the purification is in themost cases carried out by back-washing, shaking, or a combination ofthereof, by way of a short pressurized air pulse providing a shock-likedisplacement of the filter medium, at the same time as the normal gasflow is replaced by a short-time, counter-directional gas flow.

[0006] The effectiveness/efficiency of a barrier filter increases by anincreasing thickness of the heap of separated dust. A consequencethereof is that the degree of separation will decrease, when a filterelement is cleaned. Thus, it is desirable that one does not clean toooften and neither completely removes the formed dust heap. The frequencyof the cleaning as well as the intensity of the cleaning should thereforbe chosen .so that an optimal function is achieved. By optimal functionis generally meant either that the time mean value of the dust dischargebecomes as small as possible or that the energy consumption for the dustseparation is minimised under the sub-condition that a given upper limitfor the time mean value of the dust discharge is not exceeded.

[0007] A common principle of controlling is to let the operation proceedunder increasing flow resistance until the pressure drop over thebarrier filter reaches a predetermined upper limit value and then tostart a cleaning cycle implying that all filter elements, for instancefilter tubes or filter cartridges of textile material, are consecutivelycleaned, separately or in groups, whereby all receive a similartreatment. After a completed cleaning cycle, the pressure drop issmaller and, thereafter, one waits until the pressure drop, due to theincreasing heap of dust, reaches the predetermined upper limit value, atwhich the next cleaning cycle is started. Since the pressure drop is notonly dependent on the filter medium with dust heap, but also increaseswith the gas flow, one usually regards resistance as a common term forpressure drop or pressure drop corrected with regard to the volume flowof the gas. Henceforth, resistance referring to this extended meaning isused.

[0008] As an alternative, the cleaning cycle can be interrupted when theresistance has decreased with a predetermined difference or reaches apredetermined lower limit value. In such cases, the interrupted cleaningcycle is continued when the pressure drop again reaches the upper limitvalue, so that the cleaning frequency becomes the same for all filterelements.

THE OBJECT OF THE INVENTION

[0009] The main object of the present invention is to devise a methodfor determining the frequency and intensity for cleaning barrier filtersin order to achieve an optimal function, which generally implies aimingat the lowest time mean value for the emission of dust.

[0010] A second object is to devise a method for determining thefrequency and intensity for cleaning barrier filters, providing anincreased life for the filter elements relatively to known strategies ofcleaning.

[0011] A third object is to devise a method for determining thefrequency and intensity for cleaning barrier filters, allowing anindividual adaption of the cleaning for separate filter elements orgroups of filter elements in dependence of the dust load for theparticular separate filter element or the actual group of filterelements, and thereby dynamically follow changed operational conditions.

SUMMARY OF THE INVENTION

[0012] The present invention refers to a method for cleaning a barrierfilter, comprising a plurality of filter elements, of woven fabric orfelt, arranged to separate particles out of a polluted gas. The filterelements are cleaned, separately or in groups, by pressurized airpulses, the frequency, the maximum pressure, and the duration of whichcan be varied by an adjustment in order to minimise the total emissionof dust and maximise the life of the filter elements.

[0013] In the method according to the invention, the frequency and/orthe maximum pressure and/or the duration of the pressurised air pulsesare varied for a separate filter element, for a group of filterelements, or a plurality of groups of filter elements. After each pulse,the maximum value for the instantaneous emission, the emission peak, isdetermined, and the emission peak is used, after cleaning a certaingroup of filter elements, for selecting the frequency and/or the maximumpressure, and/or the duration of the pulses for this group of filterelements during continued operation.

GENERAL DESCRIPTION OF THE INVENTION

[0014] In the ideal picture of a barrier filter, all dust is caught onthe surface of the filter elements facing the dust-containing raw gas.However, in practice some dust penetrates into the filter material,usually a felt, and a small share thereof passes through.

[0015] The cleaning of barrier filter elements, in the form of tubes,bags, or cartridges, where the dust-containing gas streams from outsideand into the element by pressurised air pulses, has to be done withregard to several side effects. With the object to achieve as lowemission of dust as possible, one should allow a certain thickness ofthe dust heap on the filter element. This improves the separation, butas a negative consequence it provides an increased resistance and thusan increased energy consumption. In order to prevent the emissionimmediately after cleaning from being too large, one does not want toremove the entire dust heap in connection with the cleaning. This sets alimit for the size of the pressurised air pulses (cleaning pulses).

[0016] When the pressurised air pulse; during the cleaning, rushes as apressure wave along the filter material, the latter is moved under largeacceleration in a direction opposite to the normal gas flow. Themovement is deccelerated abruptly, when the filter material is stretchedout and thereafter a reversed movement occurs, which is interrupted whenthe filter material is stretched against the basket or the like holdingthe filter element stretched during the operation. At the seconddecceleration, forces of inertia lead to the fact that remaining dustpenetrates deeper into the filter material and the amount which therebypasses through the element provides a noticeable short-time emissionincrease.

[0017] The size of the pressurised air pulse influences this short-timeemission peak. Thus, the emission peak can give qualitative informationabout how much dust that is moved in an undesired direction in thefilter material in connection with the cleaning. Thereby, an indicationof the degree of clogging in the filter material is obtained and of theclogging velocity as well. The size of the mission peak here comprisesthe maximum value of the dust emission as well as the difference betweenthe maximum value of the dust emission and the value of the dustemission just before the cleaning pulse.

[0018] According to the present invention, it is therefore suggestedthat one uses the size of the emission peak after cleaning as anindicator of the suitable size of the pressurised air pulse, which wasused for cleaning. This can be adapted to every single filter element orto a group of filter elements depending on the construction of thedistribution system, which introduces pressurised air pulses into thefilter elements of the barrier filter. In a tubular filter, thisgenerally implies that the cleaning is done in rows and that thesmallest group is constituted of one row of tubes.

[0019] In the method according to the invention, the frequency and/orthe maximum pressure and/or the duration of the pressurised air pulsesare varied for a separate filter element, for, a group or a plurality ofgroups of filter elements. Henceforth, by a group of filter elementsalso one single element is meant. By duration also the actual time lapseof the pulse is comprised, i.e. how fast it increases and how fast itdecreases. After each pulse, the maximum value for the instantaneousemission of dust, the emission peak, is determined and the emission peakis used, after cleaning of a certain group of filter elements, forselecting the frequency and/or the maximum pressure and/or the durationof the pulses for this group of filter elements during the continuedoperation. Suitably, this selection is carried out in such a manner thatthe pulse parameter combination, which for the actual group of filterelements provides the lowest emission peak, is selected. Thereby certaingiven sub-conditions should be taken into consideration.

[0020] The cleaning frequency can suitably be determined in aconventional way so that the cleaning is done when the pressure dropover the barrier filter reaches a predetermined maximum value, dependingon filter material and dust character, for instance between 1000 Pa and2000 Pa, preferably between 1200 Pa and 1600 Pa.

[0021] When a determined maximum value is reached, one or a plurality ofgroups of filter elements is/are cleaned consecutively until thedifference between the maximum value and the noticed pressure drop overthe barrier filter amounts to a predetermined value, for instance 20-100Pa, preferably 30-70 Pa. At this value, the cleaning cycle isinterrupted and is resumed when the pressure drop again reaches thepredetermined maximum value.

[0022] In order to prevent that the life of the filter elements isnegatively affected by the actual adjustment, the maximum pressure ofthe cleaning pulses, during the adjustment as well as during operation,should be kept over a predetermined limit value. This limit value shouldbe chosen dependent on the degree of clogging in the filter material sothat one, in connection with an optimal cleaning without interruption,carries out a complete cleaning cycle, i.e. cleans all filter elementsin the barrier filter, without reaching the desirable change of thepressure drop, increases this limit value. This can, for instance, bedone by increasing the pressure in the pressure tank, from which the airto the pressure pulse is supplied.

[0023] Furthermore, it can be appropriate to measure the volume flow forthe gas to be purified and in applicable cases the first and the secondlimit value as well as the desirable change of the pressure drop areadapted to the actual volume flow by defining limit values for theresistance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The invention is now to be described in detail with reference tothe appended drawings, where

[0025]FIG. 1 discloses a schematic side-view of a tubular filter with adevice for pressurised air pulse cleaning and control equipment adaptedfor carrying out the method according to the invention.

[0026]FIG. 2 discloses schematically a view from above of the sametubular filter but without the control equipment.

DESCRIPTION OF PROPOSED EMBODIMENT

[0027] In FIG. 1 and FIG. 2, a tubular filter 1 with a housing 2, aninlet 3 for the gas to be purified, and an outlet 4 for the purified gasare disclosed. The tubular filter 1 is divided into a raw gas chamber 5for the incoming gas and a pure gas chamber 6 for the outgoing gas by anintermediate wall 7.

[0028] The intermediate wall 7 supports four rows 40, 30, 20, 10, eachhaving four filter tubes 142.

[0029] To the tubular filter 1, a system 8 is connected for cleaning thefilter tubes 142 by means of pressurized air pulses. To this purposeeach row 40 etc. of tubes 142 is provided with a distributing pipe 140having a nozzle 141 located centrally above each tube 142. For each row40, 30, 20,10 there is a separate valve member 14, 13, 12, 11 on thedistributing pipe 140.

[0030] A pressurised air tank 81 is via a first control member 93connected to a not shown overpressure source, for instance a compressor,and via a second control member 92 connected to said valve members 14etc.

[0031] For measuring of actual parameters, there are measuringtransducers 94 for the pressure in the raw gas chamber 5, measuringtransducers 95 for the pressure in the raw gas chamber 6, and measuringtransducers 96 for the dust concentration in the outlet 4, and possiblyalso a (not shown) measuring transducer for the gas volume flow. Thecontrol members 93, 92 and the valve members 14 etc. are controlled by acontrol apparatus 9 based on signals from the measuring transducers 94,95, 96.

[0032] In the method according to the invention, dust-containing gasstreams in through the inlet 3 to the raw gas chamber 5 through thetubes 142 and the pure, gas chamber 6 via the outlet 4 to a not shownchimney. The pressure in the raw gas chamber 5 and in the pure gaschamber 6 is measured, substantially continuously, by the measuringtransducers 95 and 96. During the operation dust which build up a dustheap on the outer sides of the tubes 142 are separated. When thethickness of the dust heap increases, the pressure drop increases aswell. When the pressure difference between the raw gas chamber 5 and thepure gas chamber 6 reaches a first predetermined limit value, forinstance 1400 Pa, a row of tubes is cleaned. The new pressure differenceis registered. If the pressure difference after the cleaning has fallenless than 50 Pa, another row of tubes is cleaned. This is repeated untilthat value is achieved. Then, the cleaning is interrupted to be resumedwhen the pressure drop over the tubes and dust heap, i.e. the pressuredifference between the raw gas chamber 5 and the pure gas chamber 6again reaches 1400 Pa. At this occasion, the tube rows, which were notcleaned at the previous cleaning occasion, are cleaned in the samemanner as outlined, and so on.

[0033] The method according to the invention corresponds to thedescription above. The particular subject matter of the invention isthat, during an adjustment, the size of the cleaning pulses is varied,for instance by varying the pressure in the pressurised air tank 81, andthat the dust concentration in the outlet 4 is measured substantiallycontinuously at least at the cleaning occasion by the measuringtransducer 96. In this way, for each individual row of tubes 10 etc.,one seeks to determine the pulse size, which provides the lowestemission peak after the cleaning pulse, and uses this pulse size for thecontinued operation.

1. A method for cleaning a barrier filter, comprising a plurality offilter elements, of woven fabric or felt, arranged to separate particlesout of a polluted gas, whereby the filter elements are cleaned bypressurised air pulses, the frequency, the maximum pressure, and theduration of which can be varied in order to minimise, by an adjustment,the total emission of dust and maximise the life of the filter elements,characterised in that the frequency and/or the maximum pressure and/orthe duration of the cleaning pulses are varied for a separate filterelement, for a group of filter elements or a plurality of groups offilter elements, that the maximum value for the instantaneous emissionof dust, the emission peak, is determined after each cleaning pulse, andthat the emission peak is used, after cleaning of a certain group offilter elements, for selecting the frequency and/or the maximum pressureand/or the duration of the cleaning pulses for this group of filterelements during continued operation.
 2. A method according to claim 1,wherein the pressure drop over the filter unit is measured substantiallycontinuously, characterised in that the groups of filter elements arecleaned in a predetermined order and that cleaning of a group occurswhen the resistance or the pressure drop over the filter reaches a firstpredetermined limit value.
 3. A method according to claim 1 or 2,characterised in that the maximum pressure of the cleaning pulse ismaintained over a second predetermined limit value during theadjustment.
 4. A method according to claim 3, characterised in that thesecond predetermined limit value is selected in dependence of thepressure drop over the filter unit after the separate cleaning pulses.5. A method according to claim 4, characterised in that, afteraccomplished adjustment, the highest of the minimum resistances or theminimum pressure drop after the separate cleaning pulses is determinedand this highest resistance or pressure drop is used to determine thesecond predetermined limit value.
 6. A method according to claim 4,characterised in that, after accomplished adjustment, a weighted meanvalue of the minimum resistances or the minimum pressure drops after theseparate cleaning pulses is determined and this mean value is used todetermine the second predetermined limit value.
 7. A method according toany one of the preceding claims, characterised in that, for thecontinued operation, a combination of frequency and/or maximum pressureand/or duration is selected for the cleaning pulses to a group of filterelements proximate to the one which, during the adjustment, for thisgroup of filter elements provided the lowest emission peak.
 8. A methodaccording to any one of claims 2-6, characterised in that, for thecontinued operation, a combination of maximum pressure and/or durationis selected for the cleaning pulses to a group of filter elementsproximate to the one which, during the adjustment, for this group offilter elements provided the lowest emission peak.
 9. A method accordingto any one of claims 2-8, characterised in that the first predeterminedlimit value is set between 1000 and 2000 Pa, preferably between 1200 and1600 Pa.
 10. A method according to any one of claims 3-9, characterisedin that the second predetermined limit value is set between 3 and 5 bar,preferably between 3 and 4 bar.